Tried to copy this packages to /etv/portage/package.mask, but the same thing happens.
Tried to delete this blocking packages with emerge --depclean <...> but the same thing happens.
I will be very grateful for the help.

Welcome to Gentoo!
The problem here is that you can't install gnome 2 and gnome 3 at the same time because they conflict. Basically, they need the same files for their own purposes so they can't be installed side-by-side.

I believe Cinnamon is actually gnome 3 with a new look, so you may want to remove gnome 2. That will remove the block. You also might want to install another wm like fluxbox so you aren't left at the command line when things break._________________First things first, but not necessarily in that order.

The Doctor, thank you for responding. Tell me please what to prescribe to start cinnamon after installing in file ~/.xinitrc (for example - for gnome - echo "exec gnome-session" > ~/.xinitrc and startx). And whether you can use both as gnome and fluxbox?